Statistical Evaluation and Optimization of Sol-Gel Dip Coating Deposition Parameters of Multilayered Conversion Coatings for Anticorrosive Alumina Film

Multilayered conversion coatings (MCCs) can be produced on carbon steel via sol-gel dip coating to improve the corrosion resistance of alumina film/conversion coating/carbon steel systems in a saline medium. Nonetheless, the deposition parameters used to produce the MCCs can affect this performance, and the optimum conditions are generally found using univariate studies. In this work, Silica/boehmite (SB) and boehmite/silica (BS) MCCs were produced on carbon steel via sol-gel dip coating, using a central composite experimental design to evaluate the effects of deposition time (t), substrate removal speed (v), and heat treatment time (HT) on corrosion resistance of the complete system in a saline medium. This unprecedented statistical study allowed us to determine optimal MCC deposition conditions for producing coating systems with superior anti-corrosive properties. v and HT had the most significant impact on anticorrosive performance. The SB-MCC system, optimized at t = 116 s, v = 368 mm/min, and HT = 70 min, showed the best corrosion resistance (Rg = 81.6 kΩ cm2). Improved surface roughness and better coverage of sharp structures enhanced alumina adhesion, increasing stability in saline environments compared to BS-MCC/alumina.